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Stockholders' Equity (Tables)
 9 Months Ended
Sep. 30, 2020
Equity [Abstract]  
Share-based Payment Arrangement, Option, Activity
A summary of the status of stock option activity for the nine months ended September 30, 2020 is as follows (in thousands, except price data):
SharesWeighted-average
exercise price
per share
Outstanding at December 31, 2019944 $30.63 
Granted141 93.15 
Exercised(279)21.07 
Forfeited(12)43.34 
Outstanding at September 30, 2020794 $44.89 
Schedule of Nonvested Share Activity
A summary of the status of restricted stock unit activity for the nine months ended September 30, 2020 is as follows (in thousands, except price data):
SharesWeighted-average
grant date fair value
Non-vested December 31, 2019786 $41.88 
Granted227 97.33 
Vested(115)25.82 
Forfeited(18)58.11 
Non-vested at September 30, 2020880 $58.03 
Compensation Expense Related to Stock-Based Compensation Plans
The expense related to the Company’s stock-based compensation plans included in the accompanying Consolidated Statements of Income was as follows (in thousands):
Three Months Ended
September 30,		Nine Months Ended
September 30,
2020201920202019
Cost of sales$539 $265 $1,232 $807 
Research and development1,032 573 2,540 1,682 
Sales and marketing1,580 827 4,218 2,850 
General and administrative2,402 1,459 6,571 4,745 
Total stock-based compensation expense$5,553 $3,124 $14,561 $10,084 
Estimated Fair Value of Each Stock Option Award
The estimated fair value of each stock option was determined on the date of grant using the Black-Scholes option valuation model with the following weighted-average assumptions for the option grants.
Nine Months Ended
September 30,
20202019
Risk-free interest rate1.20 %2.51 %
Expected option life (in years)5.135.68
Volatility rate40 %39 %
Dividend rate0 %0 %
Weighted-average grant date fair value$35.28 $23.67